1. Field of the Invention
The present invention relates generally to magnetic memory elements having magnetic tunnel junctions (MTJ) and particularly to improving the ease of switching of the free layer of the MTJ to reduce the requisite voltage and current for causing the free layer to switch magnetic states.
2. Description of the Prior Art
Magnetic random access memory (MRAM) is rapidly gaining popularity as its use in replacing conventional memory is showing promise. Magnetic tunnel junctions (MTJs), which are essentially the part of the MRAM that store information, include various layers that determine the magnetic behavior of the device. An exemplary MTJ uses spin torque transfer to effectuate a change in the direction of magnetization of one or more free layers in the MTJ. That is, writing bits of information is achieved by using a spin polarized current flowing through the MTJ, instead of using a magnetic field, to change states or program/write/erase/read bits.
In spin torque transfer (STT) MTJ designs, when electrons flow across the MTJ stack in a direction that is perpendicular to the film plane or from the pinned (sometimes referred to as “reference” or “fixed”) layer to the free (or storage) layer, spin torque from electrons transmitted from the pinned layer to the free layer orientate the free layer magnetization in a direction that is parallel to that of the reference or pinned layer. When electrons flow from the free layer to the pinned layer, spin torque from electrons that are reflected from the pinned layer back into the free layer orientate the free layer magnetization to be anti-parallel relative to the magnetization of the pinned layer. Thus, controlling the electron (current) flow direction, direction of magnetization of the free layer magnetization is switched. Resistance across the MTJ stack changes between low and high states when the free layer magnetization is parallel or anti-parallel relative to that of the pinned layer.
However, a problem that is consistently experienced and that prevents advancement of the use of MTJs is the threshold voltage or current used to switch the free layer magnetization during a write. Such current and threshold voltage requirements are currently too high to allow practical applications of the spin torque transfer based MTJ.
MTJs with perpendicular anisotropy, such that the magnetic moment of the free layer and the fixed layer thereof are in a perpendicular direction relative to the plane of the film, are more appealing than their in-plane anisotropy counterparts largely due to the density improvements realized by the former. Existing perpendicular MTJ designs include a free layer whose magnetic orientation relative to a reference (“fixed”) layer, while perpendicular in direction, high coercivity field (Hc) of the free layer, at its edges, limits the reduction of the effective Hc of the free layer. Lower effective Hc of the free layer would allow easier switching of the free layer and would lower the threshold voltage and current required to switch the magnetization of the free layer.
It is noted that the foregoing problem occurs due to the inconsistent Hc throughout the free layer, as shown and discussed by way of a graph shortly. That is, perpendicular anisotropic field (Hk) of the free layer changes relative to the position within the free layer such that the center of the free layer generally has a lower Hc than the outer edges of the free layer with Hc essentially increasing from the center of the free layer to its outer edges. Accordingly, efforts to lower the perpendicular anisotropic field (Hk) of the free layer in order to ease switching result in lowering of effective Hc, undesirably increase the edge-to-center effective coercivity (Hc) ratio. The relationship between Hk and Hc are as follows:Hc=Hk−Hdemag  Eq. (1)
where Hdemag is the demagnetization field related to the magnetic moment, thickness, shape and size of the magnetic thin film
For a greater understanding of the foregoing problem, FIGS. 1-3 show a relevant portion of a prior art magnetic memory element and a graph of its effective coercivity field performance.
FIG. 1 shows the relevant portion of a prior art magnetic random access memory (MRAM) element 10, which includes a reference layer 3, also known as a fixed layer, a barrier layer 2, also known as a tunnel layer, and a free layer 1. This configuration is common and very well known in the art. The layers 1-3 are often times collectively referred to as a magneto-tunnel junction (MTJ). When an electron current is applied through the layer 3 towards layer 1, for example during a write operation, the MRAM element 10 switches states where the magnetic moment of the layer 1 changes direction relative to the magnetic moment of the layer 3, from a direction shown by the arrow 5 to a direction shown by the arrow 6. Such a change in the layer 1 is also known as a change from an anti-parallel state, where the direction of the magnetic moment of the layer 1 is opposed to that of the layer 3 to a parallel state, where the direction of the magnetic moment of the layer 1 is in the same direction to that of the layer 3. The resistance of the MRAM element 10 changes according to its state and typically, such resistance is higher when the MRAM is in an anti-parallel state than when it is in a parallel state.
Lowering the perpendicular Hk of the layer 1 would make switching of the state of the MRAM 10 easier, however, as earlier noted, the effective Hc reduction, which would significantly ease switching of the state of the element 10 is limited because of the high Hc present at the edges of the layer 1. This is better noticed by the figures to follow.
FIG. 2 shows generally a top view 7 of the layer 1 of FIG. 2 and a side view 8 of the layer 1 of FIG. 2. The layer 1 is shown to be 65 nano meters in diameter, by way of example, and 1.2 nano meters in thickness. In accordance with these measurements, the effective Hc, in kilo Orsteds, vs. the position along the diameter of the layer 1, in nano meters (nm), is shown in a graph in FIG. 3. Accordingly, FIG. 3 shows a graph of the effective Hc, represented by the y-axis, vs. the position along the diameter of the layer 1, represented by the x-axis, for the case where the perpendicular Hk (p-Hk) is equal to 14.5 kilo Oersted (kOe), shown by the line 9 and for the case where the perpendicular Hk of the layer 1 is equal to 13 kOe, shown by the line 11. As shown, the effective Hc increases going from the center of the layer 1 out to its edge and this change gradually increase and at the far edge of the layer 1. When decreasing the perpendicular Hk from 14.5 kOe to 13 kOe, the edge-to-center effective Hc ratio is undesirably increased from 1.6 to 3.0.
Thus, the need arises for decreasing the perpendicular anisotropic field of the free layer of an MRAM yet avoiding a substantial increase in the effective Hc of the MRAM in order to reduce the threshold voltage and current required to operate the MRAM.